I have an opened gallon of MMO that my brother actually bought and used very little of it. It has been in the garage, unprotected from the temperature fluctuations for probably 3-4 years. It may not be the best bottle to experiment and record data with like has been done in other topics, but how "off" could it really be? Out of the gallon, I would estimate 3.5 quarts remain. The '86 Mazda is down for an early hibernation and was thinking of putting some in the tank and running it before she sits up. Also thinking of regularly using it in the '02 Maxima and occasionally in the '11 Corolla (perhaps every OCI).